The heart is generally divided into four chambers, the left and right ventricles and the left and right atria. Blood passes from the right atrium into the right ventricle via the tricuspid valve. The atrial chambers and the ventricular chambers undergo a cardiac cycle consisting of one complete sequence of contraction and relaxation of the chambers of the heart. The term systole describes the contraction phase of the cardiac cycle during which the ventricular muscle cells contract to pump blood through the circulatory system. The term diastole describes the relaxation phase during which the ventricular muscle cells relax, causing blood from the atrial chamber to fill the ventricular chamber. After completion of the period of diastolic filling, the systolic phase of a new cardiac cycle is initiated.
Through the cardiac cycle, the heart is able to pump blood throughout the circulatory system. Effective pumping of the heart depends upon five basic requirements. First, the contractions of cardiac muscle must occur at regular intervals and be synchronized. Second, the valves separating the chambers of the heart must fully open as blood passes through the chambers. Third, the valves must not leak. Fourth, the contraction of the cardiac muscle must be forceful. Fifth, the ventricles must fill adequately during diastole.
When functioning properly, the human heart maintains its own intrinsic rhythm based on physiologically-generated electrical impulses. However, when contractions of the heart are not occurring at regular intervals, or are unsynchronized, the heart is said to be arrhythmic. During an arrhythmia, the heart's ability to effectively and efficiently pump blood is compromised. Many different types of arrhythmias have been identified. Arrhythmias can occur in either the atria or the ventricles. Arrhythmias may be the result of such conditions as myocardial infarction, cardiomyopathy or carditis.
Ventricular fibrillation is an arrhythmia that occurs in the ventricles of the heart. In ventricular fibrillation, various areas of the ventricle contract asynchronously. During ventricular fibrillation the heart fails to pump blood. If not corrected, the failure to pump blood and thereby maintain the circulation can have fatal consequences.
Ventricular tachycardia is an arrhythmia that occurs in the ventricular chambers of the heart. Ventricular tachycardias are typified by ventricular rates between 120–250 beats per minute and are caused by electrical or mechanical disturbances within the ventricles of the heart. During ventricular tachycardia, the diastolic filling time is reduced and the ventricular contractions are less synchronized and therefore less effective than normal. If not treated quickly, a ventricular tachycardia could develop into a life-threatening ventricular fibrillation.
Supraventricular tachycardias occur in the atria. Examples of these include atrial tachycardias, atrial flutter and atrial fibrillation. During certain supraventricular tachycardias, aberrant cardiac signals from the atria drive the ventricles at a very rapid rate.
Sudden cardiac death (SCD) may be a consequence of cardiac rhythm abnormalities occurring in the ventricles or the atria such as ventricular fibrillation, ventricular tachycardia or one of the supraventricular tachycardias. Sudden cardiac death fatally afflicts about 300,000 Americans each year.
Patients with chronic heart disease can receive implantable cardiac devices such as pacemakers, implantable cardioverter-defibrillators and HF cardiac resynchronization therapy devices. Implantable cardioverter-defibrillators (ICDs) are used as conventional treatment for patients whose arrhythmic conditions cannot be controlled by medication. These devices provide large shocks to the heart in an attempt to revive a patient from a cardiac rhythm abnormality that may result in an SCD occurrence. At the present there are no firm predictors for SCD within these devices.